A conventional example is shown in FIG. 4 and elucidated.
Numeral 1 designates an AC power source, numeral 6 designates a sewing machine control circuit, numeral 7 designates a rectifying-smoothing circuit, numeral 9 designates a solenoid drive circuit, numeral 10 designates a solenoid, numeral 11 designates a transformer, and numeral 12 designates a chopping control circuit.
With regard to the sewing machine drive apparatus constituted as mentioned above, its operation is described referring to FIG. 5.
The AC power source 1 is transformed to a rated input voltage of the solenoid 10 by the transformer 11. The output of transformer 11 is smoothed by the rectifying-smoothing circuit 7, thereby a solenoid power source is made. A solenoid drive signal of the sewing machine control circuit 6 is received by the solenoid drive circuit 9. The solenoid 10 is driven by taking said solenoid power source as an input. After an elaspe of a time period, a chopping signal of a certain frequency from said sewing machine control circuit 6 is received by the chopping control circuit 12; and thereby, output voltage of the solenoid 10 is controlled and output current of the solenoid is reduced.
However, in the convention power source above, after driving and holding the solenoid, when chopping control is executed on the output voltage by the chopping control circuit 12 for holding the solenoid by the chopping control circuit 12, an acoustic noise of chopping sound (frequency of the chopping signal) is generated giving unpleasant feeling to a user.